There are several known methods for drying aqueous solutions, slurries, solid suspensions of materials, and the like. Such known methods include convection drying, spray drying, and superheated steam drying, for example.
Spray drying is a common processing operation whereby solids dissolved in an aqueous solution or present in the form of a slurry, for example, are dried to a finely divided powdery product. Conventionally, the process includes atomizing the solution or slurry in the form of finely divided droplets which are then evaporated and dried in contact with a current of hot air at elevated temperature which acts as the drying medium.
In convection drying, heated air flows through the dryer to contact the wet feed. The water from the feed evaporates into the air, which cools the air while increasing its humidity. After the dried product, typically a powder, is separated from the air, the moist air is exhausted to the atmosphere along with some of the energy used during drying.
The convection dryer's efficiency can be improved by recycling some of the moist exhaust air together with its energy (in the form of waste heat) back to the dryer inlet. Recycling the exhaust air increases the humidity in the dryer which, in turn, requires a higher operating temperature and results in a higher final product temperature than in a completely open-circuit convection-drying operation.
Some of the convection waste heat can be recovered by passing the hot exhaust air through a heat-recovery system to warm the dryer's fresh makeup air. The heat-recovery system can be an indirect system, such as an air-to-air heat exchanger with a coil system with two recirculating, fluid-filled coils that transfer heat to the makeup air passing between them. The heat-recovery system can also be a direct system in which the makeup air is passed through a scrubber system that sprays with hot water condensed from the dryer's re-circulated exhaust air. This water can run very closely to the exhaust air's saturation temperature (the dew point), which is frequently in the range of 130° F. to 150° F. (55° C. to 65° C.). However, this temperature is limited by the amount of air present in the dryer exhaust: the more air in the exhaust, the lower the saturation temperature.
Superheated steam can be used instead of hot air for drying an aqueous solution or slurry. Steam atmosphere drying recycles the exhaust stream in a closed-circuit operation. Steam-atmosphere drying increases the temperature of the heat recovered from the dryer's exhaust by reducing the amount of non-condensible gases in it. Heat is transferred to the dryer indirectly using a fired heat exchanger, rather than directly. Indirect heat transfer allows the recirculated exhaust stream to be reheated without introducing anymore non-condensible gases. The primary air that originally fills the dryer can be purged and replaced with evaporated vapor, typically steam, to form an atmosphere of superheated vapor, or superheated steam. As with a conventional boiler, the temperature of this vapor, and thus the heat recovery, depends upon the dryer's operating pressure. Accordingly, the steam-atmosphere dryer has a higher operating temperature and a higher product temperature than a typical convection dryer.